An IGMP (Internet Group Management Protocol) is known as a multicast communication protocol realizable in an IP network. The IGMP is a protocol for determining whether or not a router sends out multicast information to a sub-network, for the purpose of avoiding congestion of a network. An access communication system connected to the network which supports the IGMP should employ a control procedure which has an affinity with the IGMP.
In a communication system according to the IGMP, while the router distributes (broadcasts) multicast information to each host in the sub-network, the router broadcasts query periodically. The host which received the query monitors as to whether a response from another host is received within a random time. When a response from another host is received by the host, this host maintains a reception condition for multicast information. However, no response from another host is received by the host, this host transmits a response (report) to the router and the other host so as to continue reception of multicast information. Then, when the router receives a response (report) from any host in response to the periodically broadcast query, the router continues distribution (broadcast) of the multicast information to each host.
Such a processing procedure may be applied to a radio section as it is. In this case, as shown in FIG. 1, a message corresponding to the above-mentioned query is transmitted to all the radio terminals present in a service area by a radio base station. The radio terminal which has received the message transmits a request for the service continuation as a response thereto to the radio base station after a random time has elapsed, when desiring continuation of multicast information distribution service. When receiving a response to the above-mentioned query from any radio terminal, the radio base station continues the multicast information distribution service.
Thus, in case the processing procedure of IGMP is applied to the radio section, when receiving the message corresponding to the query from the radio base station, the radio terminal can transmit a response thereto to the radio base station. However, it is not possible to transmit another radio terminal in case of IP network. If the above-mentioned response transmitted from the radio terminal could not be received by all the other radio terminals which receive multicast information in the same service area, the response would be returned to the radio base station from all the radio terminals which desire continuation of receiving the multicast information. Thereby, congestion is likely to occur in the radio section.
Further, an access to a channel transmitting the above-mentioned request for service continuation occurs randomly from a plurality of radio terminals. However, in case the permissible number of the accesses is limited for this channel, collision occurs when many radio terminals simultaneously transmit the requests for service continuation by chance. Thereby, as shown in FIG. 1, the request for service continuation does not reach the radio base station (see a sign ×). When the radio base station operates in accordance with a protocol such as IGMP, the radio station terminates multicast information distribution service after a predetermined time has elapsed without transmission of a request for service continuation made from any radio terminal after transmitting the query. Thereby, the service would be interrupted even though there are the radio terminals which request service of distributing multicast information in the service area if the request for service continuation made from the many radio terminals simultaneously by chance collided as mentioned above.
Further, recently, verification for a multicast application performing broadcast-like distribution of music or movies via the Internet employing a cable network, or performing cooperative works or remote meeting by a plurality of users has been performed by using IGMP performing group management of various multicast service.
On the other hand, through spread of portable telephone terminals such as cellular phones, PHS terminals, or the like, or portable information terminals such as so-called notebook personal computers, it is expected that a demand of radio provision of multicast service increases. When an PC terminal of an end user of a network and a router of a sub-network in which this PC terminal exists can operate in accordance with IGMP, provision of multicast service can be made whether the channel is of cable or radio.
Therefore, a system such as that shown in FIG. 2 can be assumed as a system performing the above-mentioned provision of multicast service, for example. This example employs a radio base station 220 which can operate in accordance with IGPM which acts as an access point for a radio LAN. In this case, multicast information A, B and C provided by various servers 251, 252 and 253 via an IP network NW and a router 230 is distributed from the radio base station 220.
For example, when radio terminals 210(1), 210(2) and 210(3) including radio LAN connection communication devices belonging to a service area Es (LAN) of radio LAN and PC terminals request multicast information A, B and C different from each other, the radio base station 220 broadcasts all the multicast information A, B and C thus requested. Then, each of the radio terminals 210(1), 210(2) and 210(3) receives all the multicast information A, B and C. Then, in each radio terminal (PC terminal), necessary multicast information is extracted from the thus-received information to be used.
In case the radio base station 220 distributes multicast information in the service area Es (LAN) of radio LAN, no individual radio channel is set for each radio terminal. However, a data stream including the plurality of sets of multicast information broadcast from the radio base station 220 mixed therein is received by each radio terminal (cellular phone and PC terminal), and is stored thereby, and, then, only necessary information is extracted therefrom. Therefore, when each radio terminal receives a large amount of multicast information, a heavy burden is loaded thereon for buffering it.
Furthermore, a system such as that shown in FIG. 3 can also be assumed as a system performing the above-mentioned radio provision of multicast service, for example. This example performs multicast service by using a radio base station 220 connected to a public network NW such as an existing PDC (Personal Digital Cellular) or PHS. In this case, multicast information A provided from a server 250 via the public network NW is distributed from the radio base station 220 to respective radio terminals 210(1), 210(2) and 210(3) belonging to a service area Es thereof. In distributing the multicast information A, an individual channel is set between each of the radio terminals 210(1), 210(2) and 210(3), and the radio base station 220.
In such a system in which multicast service is performed by using the base station 220 connected to the public network NW, the individual radio channel should be set for each radio terminal even for transmitting the same multicast information within the same service area Es. Accordingly, it is not possible to effectively utilize the radio resources.
Furthermore, a system shown in FIG. 4 can also be assumed for performing the above-mentioned radio provision of multicast information, for example. This example performs multicast service in an advanced radio calling system (FLEX-TD). In this system, a radio base station 220 transmits multicast information provided from various servers 251, 252 and 253 via a public network NW by using a radio channel set for each set of multicast information regardless of whether or not radio terminals which desire the service belonging to a service area (radio zone) Es exist, in a broadcast-like manner. Each of the radio terminals 210(1), 210(2) and 210(3) can receive multicast information for which contract has been made previously. For example, the radio terminal 210(1) which made a contract for multicast information A can receive only the multicast information A from among multicast information A, B and C, the radio terminal 210(2) which made a contract for multicast information A and C can receive only the multicast information A and C, and the radio terminal 210(3) which made a contract for multicast information C can receive only the multicast information C.
However, in such a system, each radio terminal can receive only multicast information for which a contract has been made. However, even when the radio terminal which made a contract of receiving multicast information does not exist within the service area Es, the radio base station 220 should perform continues transmission of the multicast information. Thus, it is not possible to effectively utilize the radio resources.
Furthermore, when information multicast distribution service is realized in a radio communication system, in particular, in a mobile communication system, smooth switching of base station from which information is received by a mobile set is needed when the mobile set which is a radio terminal moves across service areas (in case of handoff) while receiving the multicast distribution of the information.
Thus, in case information multicast distribution service is applied to a radio communication system, various problems should be solved.